Serial bus systems are being used more and more for production and process automation, wherein the decentralized devices of peripheral machines, such as I/O modules, measurement transducers, drives, valves, and operator terminals, communicate by means of a powerful real-time communications system with automation, engineering, or visualization systems. In this way, all instruments are linked to each other by means of one serial bus, preferably a field bus, wherein the data exchange by means of the bus is usually performed on the basis of the master-slave principle.
Here, the active stations on the bus system are the control devices. They are in possession of a bus-access token and determine the data transfer on the bus. The active stations are also called master devices in the serial bus system.
In contrast, passive stations are usually the peripheral machines. They receive no bus-access token, i.e., they are allowed only to acknowledge received messages or transmit messages to active stations upon a request by this active station. Therefore, passive stations are also called slave devices in the serial bus system.
In general, the master devices contain a field-bus switch-on device, which represents the link between the central data processing in the master device and the field-bus network, and which performs bus management. Thus, the switch-on device that is often configured as a separate assembly also implements the complete bus protocol. The slave devices in turn feature an interface module, which converts the data from the slave device into the data format of the field-bus system. Therefore, these interface modules also require only a small portion of the bus protocol.
In automation technology, field-bus systems are used according to the master-slave principle with a wide variety of different transmission rules. For cyclical field-bus systems, e.g., Profibus-DP, ControlNet, FIP-IO, or Interbus-S, the data is transmitted in a cycle independent of whether the data has been changed. In contrast, in acyclical field-bus systems, like those known for CAN systems, data is only transmitted if the data has been changed or if the data transmission is explicitly triggered by the master device.
Further distinctions are also made between stationary-oriented field-bus systems, like Profibus-DP, for which a master device sends a message to a slave device that then acknowledges or answers this message, and message-oriented field-bus systems, like the CAN system, which is distinguished in that the master device outputs unacknowledged messages onto the bus, which can then be processed by all stations. Furthermore, there are also bus-oriented field-bus systems, like Interbus-S, for which the master device transmits a message with all of the data for all of the attached slave devices, wherein the site of the data for the respective slave device is determined by its position In the message block.
Control processes, especially for automation systems used in production, are usually assembled from one or more tasks, which are in general performed in a cycle. Such tasks are performed in a field-bus system so that the slave devices, which represent the peripheral machines, deliver input process data by means of the field bus to the master device, which functions as a process controller. In the master device, the output process data is then generated corresponding to the task to be performed, and is transmitted by the field-bus system to the slave devices.
In the conventional transfer of process data, the process data of the slave devices is output according to the bus protocol used by the interface unit of the appropriate slave device onto the field bus, and received by the field-bus switch-on device as the interface of the master device with the field bus. The field-bus switch-on device in the master device then builds a process image for the appropriate task from the received process data of the slave devices, and forwards this process image to the data processing unit in the master device. The data processing unit in the master device in turn creates an output process image, which is output to the field-bus switch-on device, on the basis of the input process image according to the task to be performed. The field-bus switch-on device then determines the process data for the individual slave devices from this process image and transmits this process data according to the provided bus protocol to the appropriate slave device by means of the field bus.
For the known field-bus systems, the process data transfer between the master device and the slave devices is performed conventionally, so that the process data for the individual process devices is assembled into separate data packets, which are also called telegrams in the following, and the slave devices are then addressed individually.
For the process data transfer, the field-bus switch-on device in the master device must therefore perform a complex allocation between the process data transmitted from and to the slave devices and the input and output process images to be allocated to each task. However, this allocation, which is also called mapping in the following, of process data of the slave devices to the process images, which are used in the master device and which are formed in the field-bus switch-on device of the master device, requires a high processing effort. Therefore, a complex mapping algorithm and protocol sequence must also be installed in the field-bus switch-on device. This applies even more to open field-bus systems, which are designed so that stations can be removed from the overall system and inserted into the system without great expense.
The required conversion of the process data of the individual slave devices into process images in the field-bus controller also leads to significantly delayed processing of processes, especially when several tasks are supposed to be performed simultaneously.
This applies above all when a control application consists of several partial processes, each of which is allocated to a different slave device. For conventional field busses, the process image allocated to the control application is then divided into corresponding individual telegrams for the appropriate slave devices. In contrast, in the event that the process data for one task has to be made available to several slave devices, because all of these slave devices are supposed to perform this task, the field-bus controller must then build a unique telegram with the process data for each slave device.
Furthermore, if the individual partial processes have different cycle times, conventional data transmission methods on the field bus cannot adapt the bus load to the individual partial processes according to the different cycle times, and thus cannot achieve an optimal use of the bus.
In addition, conventional field-bus systems can only with much difficulty provide flexible adaptation of the telegrams to the process data lengths required in the slave devices. Thus, if a slave device requires only 1-bit process data, in general an additional bus coupler with a local bus network is necessary in order to convert this 1-bit process data into the telegrams of the standard field bus.
The problem of the present invention is to refine the known serial bus systems, especially field-bus systems, so that flexible data transmission is possible with reduced data processing effort.
This problem is solved by a method according to claim 1, a serial bus system according to claim 7, and a switch-on unit according to claim 11. Preferred embodiments are given in the dependent claims.